In integrated sensors having three terminal lines VDD (positive supply voltage pin), GND (negative supply voltage pin) and OUT (output voltage pin), wherein the measured quantity of the sensor is output at OUT in the form of a mostly analog output voltage, there is occasionally used an OBD system (open bond system). As, in many cases, for example in vehicle applications, sensors are connected to an evaluating unit via long lines, it may occur that one of these lines is opened (line interruption) or is short-circuited to another line. In order to be able to detect an opened line, there is the OBD system. All terminals on a chip are connected to normally-on structures. If the integrated circuit (IC) contained on the chip is in normal operation, these normally-on structures are switched high-ohmic and do not affect the operation, at least not substantially. If, however, one of the supply lines breaks, the normally-on structures become low-ohmic and connect the remaining supply pin to the output voltage pin (OUT pin). In such a case, an evaluating unit detects an error due to the voltage at the output pin, because, in normal operation, a valid voltage at the output voltage pin (OUT voltage) is prevented from coming too close to one of the two supply potentials. If the voltage at the output voltage pin (OUT) is close to the voltage VDD at the positive supply voltage pin or close to the voltage GND at the negative supply voltage pin, this condition will be described in the following by the term “the output voltage OUT is in an error band”, as by convention.
As the described normally-on structures are directly at the pins, they may be used at the same time to realize further properties of the integrated circuit (IC). In particular, this includes a reverse-connect protection (or polarity inversion protection) at the positive supply voltage pin VDD and the output voltage pin OUT. A polarity inversion protection becomes operative if a potential is turned on at the respective pins which is negative with respect to the potential at the negative supply voltage pin GND. Also, an overvoltage protection may be realized which fulfills a protective function when the potential at the output voltage pin OUT is larger than the potential at the positive supply voltage pin VDD. An overvoltage at the output voltage pin OUT occurs, for example, if there is a short circuit towards a high potential at the output voltage pin OUT.
Furthermore, the normally-on structures may be used for signaling certain events. In the case of an overvoltage, for example, i.e. if the operating voltage is so large that the integrated circuit (IC) is no longer operating properly, but is not destroyed yet either, a potential close to the potential VDD at the positive supply voltage pin or close to the potential GND at the negative supply voltage pin may be output at the output voltage pin OUT. Such a potential at the output voltage pin OUT is in the error band according to the above convention.
The German patent application, application number 10314601 shows a semiconductor circuit with a protective circuit. It uses a p-channel junction field effect transistor to realize a polarity inversion protection. If the potential at a node and/or pin to which the normally-on p-channel junction field effect transistor is connected is negative with respect to a reference potential (GND), the p-channel junction field effect transistor limits the current flow in a better way than it is achieved using alternative circuit solutions, for example a protective resistor. The protective circuit is thus suitable for the protection of a positive supply voltage pin (VDD) or an output voltage pin (OUT).
WO 02/15392 A2 shows a circuitry for the detection of an error condition with respect to the operating voltage supply of a sensor. For example, the interruption of a supply line may be detected by means of the shown circuitry. The shown circuitry provides a transistor of the normally-on type connected between a supply voltage terminal and an output terminal. In a normal operating condition, a drive circuit generates a voltage at its control input that pinches off the channel. In an error condition, the channel of the transistor is low-resistance conducting. By means of the shown circuitry, the output of a linear amplifier stage may preferably be applied to the intact potential in the case of an interruption of a supply line. This allows the detection of an error condition in a downstream evaluating circuit.
U.S. Pat. No. 6,559,721 B2 shows a circuitry with an integrated amplifier. The amplifier comprises an output stage connected to a supply potential terminal and a reference potential terminal. A pair of complementary output transistors couple the amplifier to a tri-state output. In the case that one of the operating voltage feeds connected to the reference potential terminal and the supply potential terminal in a normal operating condition is interrupted, the tri-state output is put into a high-ohmic condition by the circuitry. In order to achieve this, two blocking transistors are provided whose control terminals are supplied by associated charge pump circuits. In sensor applications, in which high reliability is required, such a circuitry allows, for example, to avoid misinterpretation of measurement results in the case of disturbances. With the help of a shown circuitry, this is possible with little effort.
US 2003/0016068 A1 shows a circuitry for discharging a circuit node. It may be employed in OBD circuits. The discharge of a circuit node is done via a field effect transistor whose gate terminal, after turning off the supply voltage, is kept to such a potential that the drain source path is conductive and discharges a node to be discharged. In a normal operating condition, however, the drain source path is blocked so that the discharge circuit is deactivated and does not represent any further load.